1. Field of the Invention. The invention, in general, relates to radio frequency (r-f) transmitters, and more particularly to a frequency modulated transmitter in which the frequency is stabilized by a surface acoustic wave (SAW) device.
2. Description of the Prior Art. Frequency modulated (FM) transmitters are well known in the radio art. Recently, surface acoustic wave resonators (SAWR) have been used in r-f receivers and in amplitude modulated transmitters. See, for example, Precision L-Band Saw Oscillator for Satellite Application, by Thomas O'Shea et al which is available from Sawtek Inc., Post Office Box 1800, Orlando, Fla. 32860. Such SAW devices are very useful in high frequency applications because above about 50 megahertz the bulk crystal oscillators of the prior art must be so small that they easily break, and the frequency multipliers of the prior art consume relatively large amounts of power, especially when used in the ultra high frequency range.
Up to the time of the present invention it was thought it was not possible to make a workable FM modulated SAW transmitter. It was thought that a SAW device could not be made stable at the frequency shifts necessary in FM transmitters, and further that the SAW device would also be unstable i.e., become free running, at the power levels required to produce a useable transmission range (over 100 feet) at high frequencies (50 megahertz-1 gigahertz). Moreover, it was believed that it was not possible to modulate the SAW to the extent necessary that the signal could be received by wide band receivers and/or that a SAW transmitter with acceptable manufacturing tolerances would not be economical.